1. Field of the Invention
The present invention relates to color conversion method and apparatus for chromakey processing.
2. Description of the Related Art
A chromakey apparatus or image compositing apparatus which utilizes a color differential background (mainly in the form of blue or green screen) for broadcast and feature movie productions is often designed to process an image to be processed in a manner described below. Firstly, an object of shooting is located in front of a single colored screen as a background and is shot to produce an image material (hereinafter referred to as an xe2x80x9cimage to be processedxe2x80x9d). Apart from this, an image material to be used as a background in composition is prepared. With these image materials being inputted to the apparatus, the following steps are carried out sequentially.
(1) One color (backing color) is selected for the background or to represent the background.
(2) Two classification boundaries are set in a color space such as an RGB or YIQ color space to divide the color space into three different regions of complete backing, complete foreground and transition regions. The complete backing region need to contain the backing color. The term xe2x80x9cRGBxe2x80x9d refers to a color system of three primary colors which have most in common with the visible characteristics of human beings. The term xe2x80x9cYIQxe2x80x9d refers to a color system which is one of so-called color difference systems adapted for TV displays and transmittal of images; Y stands for a component representing brightness and I and Q are orthogonal coordinates for indicating positions on a color hue ring.
(3) Each of pixels constituting the image to be processed is checked as to its position in the color space.
(4) When the pixel is contained in the complete backing or complete foreground region, its contribution is defined to be 0% or 100%, respectively. If it is contained in the transition region, distances of the pixel to the two classification boundaries are checked to determine its contribution on the basis of its proximity or similarity to the complete regions; the contribution is greater than 0% and smaller than 100%.
(5) From each color component of the color to be processed, each color component of the backing color is subtracted and the subtractive result is multiplied by inverse of the contribution obtained in (4). Then, the backing color is added thereto to determine an essential color of the object of shooting.
(6) The product of the multiplication of the essential color and the contribution is added to the product of the multiplication of the composite background and the complement of the contribution, i.e., (100%-[contribution]).
The composite result can be obtained by the above steps. In step (2) above, an RGB space with three axes of the three primary colors of red, green and blue is popularly used in conventional apparatuses.
The conventionally used RGB space is, however, a color coordinate space adapted to characteristics of a camera or a display and has little causal relation to intrinsic processing structure of chromakey. A coordinate space should be employed which is suited for classifying colors and for checking distances in a color space. In other words, it is essential to use a coordinate space which contributes to easy handling such as checking of similarity of the pixels of the image to be processed to the backing color or to the color of the object of shooting in the foreground.
The present invention was made in view of the above and has its object to provide color conversion method and apparatus for chromakey processing which can produce a masked output image and a color-processed foreground output image without requiring complex arithmetic operations in order to produce a composite image with backing color components being suppressed.
The present invention is directed to a color conversion method for chromakey processing wherein a foreground image taken with an object of shooting located in front of a single colored screen and a background image to be used in a completed composite image are composited, which comprises
transforming an original coordinate system into a coordinate system wherein a backing vector is a principal axis and a backing color selected is at an original point, said backing vector being line drawn perpendicularly from the backing color to monochrome straight line, and
comparing a distance between pixel data to be processed and the backing color, a distance defined by a base control variable selected and a distance defined by a mask control variable selected, generating transformation coefficients on the basis of a comparison result before pixel processing and using said transformation coefficients to perform processing on a pixel by pixel basis, thereby generating a mask signal.
Said color conversion method for chromakey processing may further comprise comparing the distance between pixel data to be processed and the backing color, the distance defined by the base control variable selected and a distance defined by a spill control variable selected, generating transformation coefficients on the basis of a comparison result before pixel processing and using said transformation coefficients to perform processing on a pixel by pixel basis, thereby suppressing any spill of backing color components existing in the foreground image and replacing the same with a replacement color selected.
The present invention is further directed to a color conversion apparatus for chromakey processing wherein a foreground image taken with an object of shooting located in front of a single colored screen and a background image to be used in a completed composite image are composited, which comprises
a setup data input module for setup of a backing color, a replacement color, a base control variable, a mask control variable and a spill control variable,
a setup data translation module for transforming an original coordinate system into a coordinate system wherein a backing vector is a principal axis and a backing color selected in said setup data input module is at an original point, said backing vector being line drawn perpendicularly from the backing color to monochrome straight line, for comparing a distance between pixel data to be processed and the backing color, a distance defined by the base control variable selected and a distance defined by the mask control variable selected, for comparing the distance between pixel data to be processed and the backing color, the distance defined by the base control variable selected and a distance defined the by the spill control variable selected and for generating transformation coefficients on the basis of comparison results before pixel processing and
a pixel processing module for using said transformation coefficients generated in the setup data translation module to perform processing on a pixel by pixel basis, thereby generating a mask signal and suppressing any spill of the backing color components existing in the foreground image and replacing the same with a replacement color selected.
Color conversion method and apparatus according to the invention can provide the following advantages.
According to the invention, two concepts are adopted:
(1) conversion to arrange a group of colors contained in the object of shooting in the foreground to a color form close to a plane typically expressed by z=constant; and
(2) conversion to enable a distance between any selected color and the backing color on the plane of z=constant to be dealt with a normalized numerical value. Thus, these conversions are adopted so that the colors of the object of shooting, which need to be treated equivalently in the composition process, can be concentrated in a single spot in a one-dimensional feature space along, for example, the z-axis. The z-component represents the similarity to the backing color. Moreover, the positional relationship between the color to be processed and the classification boundaries can be computed simply by comparing them with respect to size in terms of one of the components (z component) of the space coordinate. Furthermore, the proximity to the classification boundaries can be detected simply by checking the size of one of the components of the space coordinate.
As for internal processing, the invention provides the advantage that the multiplication of the inverse of the contribution (or the division using the contribution as a divisor) in step (5) and the multiplication in step (6) can be replaced by a multiplication and an addition by carrying out the operations in a normalized coordinate space. This processing using the color space can be utilized as a core to efficiently implement functional features of chromakey such as correction of an uneven backing, processing of a semitransparent object of shooting and suppression of spill of the backing color to the object of shooting in the foreground.
Now, the invention will be described in greater detail with reference to the accompanying drawings.